List<List<string>> res = Generate("abcdefghijklmnopqrstuvwxyz0123456789-");
            Console.WriteLine("Count = " + res.Count);
            for (int i = 0; i < res.Count; i++)
            {
                Console.WriteLine(res[i]);
            }

        public static List<List<string>> Generate(string word)
        {
           List<List<string>> result = new List<List<string>>();

           List<string> dList = new List<string>();
            foreach (char i in word)
            {
                dList.Add(i.ToString());
            }
            result.Add(dList);

            int a = 0;

            do
            {
                result.Add(GetNext(word, result[a]));
                a++;
            } while (a < result.Count);
            
            return result;
        }
 

        private static List<string> GetNext(string word, List<string> list)
        {
            List<string> dList = new List<string>();

            int count = list.Count;
            for (int i = 0; i < word.Length; i++)
            {
                for (int a = 0; a < count; a++)
                {
                    dList.Add(word[i].ToString() + list[a]);
                }
            }

            return dList;
        }
 

a，b，c，……

aa，ab，ac，……
ba，bb，bc，……
ca，cb，cc，……

aaa，aab，aac，……aba，abb，abc，……aca，acb，acc，……
baa，bab，bac，……bba，bbb，bbc，……bca，bcb，bcc，……
caa，cab，cac，……cba，cbb，cbc，……cca，ccb，ccc，……

aaaa，aaab，aaac，……aaba，aabb，aabc，……aaca，aacb，aacc，……

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